I suspect that it's a temperature issue.
The presence of condensed water vapour in the exhaust indicates that the water produced by combustion has been cooled below it's saturation temperature, which is a function of absolute humidity.
A car has water cooled cylinders, which take longer to warm up than air-cooled and there is a longer length of exhaust system, allowing more heat loss before the gases get dispersed in the atmosphere.
Aero-engines are more comparable to motor-cycles in that respect.